1. Field of the Invention
An electroluminescent (EL) assembly is characterized in that it emits light on application of an electric potential with flow of electric current. Such assemblies have long been known in industry under the name light-emitting diodes (LEDs). The emission of light results from positive charges (holes) and negative charges (electrons) recombining with emission of light.
2. Description of the Prior Art
In the development of light-emitting components for electronics or optics, use is at present mainly made of inorganic semiconductors such as gallium arsenide. Display elements having a dot form can be produced on the basis of such substances. Large-area assemblies are not possible.
Apart from the semiconductor LEDs, electroluminescent assemblies based on vapour-deposited, low molecular weight organic compounds are known (U.S. Pat. Nos. 4,539,507, 4,769,262, 5,077,142, EP-A 406 762, EP-A 278 758, EP-A 278 757).
Furthermore, polymers such as poly-(p-phenylenes) and poly-(p-phenylene-vinylenes (PPV)) have been described as electroluminescent polymers: G. Leising et al., Adv. Mater. 4 (1992) No. 1; Friend et al., J. Chem. Soc., Chem. Commun. 32 (1992); Saito et al., Polymer, 1990, Vol. 31, 1137; Friend et al., Physical Review B, Vol. 42, No. 18, 11670 or WO-A 90/13148. Further examples of PPVs in electroluminesceiat displays are described in EP-A 443 861, WO-A 92/03490 and WO-A 92/003491.
Heeger et al. have proposed soluble conjugated PPV derivatives for producing flexible polymer LEDs (WO 92/16023).
Polymer blends of various compositions are likewise known: M. Stolka et al., Pure & Appl. Chem., Vol. 67, No. 1, pp 175-182, 1995; H. Bassler et al., Adv. Mater. 1995, 7, No. 6, 551; K. Nagai et al., Appl. Phys. Lett. 67 (16), 1995, 2281; EP-A 532 798.
The organic EL assemblies generally contain one or more layers of organic charge transport compounds. The in-principle structure in order of the layers is as follows:
1. Support, substrate PA1 2. Base electrode PA1 3. Hole injection layer PA1 4. Hole transport layer PA1 5. Light-emitting layer PA1 6. Electron transport layer PA1 7. Electron injection layer PA1 8. Top electrode PA1 9. Contacts PA1 10. Sheathing, encapsulation
Layers 3 to 7 represent the electroluminescent element.
This structure constitutes the most general case and can be simplified by leaving out individual layers, so that one layer assumes a plurality of functions. In the simplest case, an EL assembly consists of two electrodes between which there is an organic layer which performs all functions including the emission of light. Such systems are described, for example, in the Application WO-A 90/13148 on the basis of poly-(p-phenylene-vinylene).
Multilayer systems can be built up by vapour deposition methods in which the layers are applied successively from the gas phase or by casting methods. Owing to the higher process speeds, casting processes are preferred. However, the partial dissolution of a layer which has already been applied when the next layer is applied on top can, in certain cases, be a difficulty.
U.S. Pat. Nos. 4,539,507 and 5,150,006 describe metal complexes based on 8-hydroxyquinoline derivatives having different central atoms, e.g. Zn, Al, Mg or Li, which function as emitter and electron injection and transport layers in an electroluminescence assembly. However, the compounds described have only a low solubility in alcohols so that they can be applied only by vapour deposition methods. If these compounds were applied from solvents such as THF or 1,2-dichloroethane, the previously applied hole conductor layer would be partially dissolved or the hole conductor could be leached from the polymer matrix.
The object of the present invention is to provide electroluminescent assemblies having a high light flux, where the mixture to be applied can be applied by casting. This applies to all of the hole injection, hole transport and electroluminescent or electron transport layers. In order that the application of the electroluminescent or electron transport layer does not result in partial dissolution of the layer underneath, the compounds used have to be soluble in alcohols, particularly in methanol. It was therefore necessary to synthesize specifically substituted metal complexes which have sufficient solubility in methanol for the casting process.
It has been found that electroluminescent assemblies which contain the blend system mentioned below and also the alcohol-soluble emitters or electron conductors (metal complexes) of the present invention meet these requirements. In the following, the term "zone" is equivalent to "layer".